Conventional industrial control systems allow control of remote field devices, such as a valve positioner controlling a fluid flow. For example, such a field device may set a position of the valve ranging from “closed” to “open” (i.e., 0%–100%) based on a current value, ranging, for example, from 4–20 mA provided to the positioner. In such systems, the valve position may vary in direct relation to the current, thus being fully closed at 4 mA and fully opened at 20 mA. The current may be sent from a control system via a two-wire interface, and received at an interface circuit at the field device that converts the analog current signal to a digital signal, which is then analyzed by a processor and applied to adjust the valve position as just described.
The control system is often remote from the field device, and connected only via the two-wire interface. Therefore, conventional industrial control systems often supply power to the field device via the same two-wire interface. The available power may be, for example, a current value of the 4–20 mA control current multiplied by the loop input voltage at the input of the field device.
In addition to the 4–20 mA analog control signal, conventional industrial control systems gain additional control over the field device by adding a digital communications signal to the analog control signal already present on the two-wire interface. The digital communications signal can be used to monitor secondary variables and other data that can be used for tasks including operations, commissioning, maintenance, and diagnostic purposes.
Specifically, for example, a digital communications signal can be added to the 4–20 mA control signal using the Highway Addressable Remote Transducer (“HART”) protocol. The HART protocol uses a Frequency-Shift Key (“FSK”) modulation technique. Advantageously, the HART protocol is functional in the low-power environment provided by the use of the 4–20 mA control signal for power (resulting in available power in the mW range), and is otherwise compatible with (and therefore easily retrofitted to) existing industrial control systems.
Thus, conventional industrial control systems may provide a 4–20 mA analog control signal, a digital communications signal, and electrical power to a remote field device, all via a two-wire interface. Conflicting requirements of these three signals, along with characteristics of conventional industrial process control settings, may lead to various challenges in implementing robust industrial control systems.